The order Notostraca comprises the single family Triopsidae, containing the tadpole shrimp[1] or shield shrimp.[2] The two genera, Triops and Lepidurus, are considered living fossils, having not changed significantly in outward form since the Triassic. They have a broad, flat carapace, which conceals the head and bears a single pair of compound eyes. The abdomen is long, appears to be segmented and bears numerous pairs of flattened legs. The telson is flanked by a pair of long, thin caudal rami. Phenotypic plasticity within taxa makes species-level identification difficult, and is further compounded by variation in the mode of reproduction. Notostracans are omnivores living on the bottom of temporary pools and shallow lakes.

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Notostracans are 2–10 centimetres (0.8–3.9 in) long, with a broad carapace at the front end, and a long, slender abdomen.[1] This gives them a similar overall shape to a tadpole, from which the common nametadpole shrimp derives.[1] The carapace is dorso-ventrally flattened, smooth, and bears no rostrum; it includes the head, and the two sessile compound eyes are located together on top of the head.[1] The two pairs of antennae are much reduced, with the second pair sometimes missing altogether.[2] The mouthparts comprise a pair of uniramous mandibles and no maxillipeds.[2]

The ventral side of Triops australiensis, showing the many pairs of phyllopodous legs

The trunk comprises a large number of "body rings", which appear to be body segments, but do not always reflect the underlying segmentation.[1] The first eleven body rings compose the thorax and bear one pair of legs each, the last of which also bears the genital opening.[1] In the female, it is modified to form a "brood pouch".[3] The first one or two pairs of legs differ from the remainder, and probably function as sensory organs.[3]

The remaining segments form the abdomen. The number of body rings is variable both within and between species,[1] and the number of pairs of legs per body ring can rise as high as six.[4] The legs become progressively smaller along the abdomen,[3] with the last segments being legless.[1]

The abdomen ends in a telson and a pair of long, thin, multi-articulate caudal rami.[5] The form of the telson varies between the two genera: in Lepidurus, a rounded projection extends between the caudal rami, while in Triops there is no such projection.[1]

A collection of lake sediment containing the pink eggs of Triops longicaudatus

Within the Notostraca, and even within species, there is variation in the mode of reproduction, with some populations reproducing sexually, some showing self-fertilisation of females, and some showing a mix of the two.[1] The frequency of males in populations is therefore highly variable.[3] In sexual population, the sperm leave the male's body through simple pores, there being no penis. The eggs are released by the female and then held in the cup-like brood pouch.[3] The eggs are retained by the female only for a short time before being laid,[4] and the larvae develop directly, without passing through a metamorphosis.[2]

The "central autapomorphy" of the Notostraca is the abandonment of filter feeding in open water, and the development of a benthic lifestyle in muddy waters, taking up food from particles of sediment and preying on small animals.[4] A number of other characteristics are correlated with this change, including the increased size of the animal compared to its relatives, and the loss of the ability to hinge the carapace; although a central keel marks the former separation into two valves, the adductor muscle is missing.[4] Notostracans retain the plesiomorphic condition of having two separate compound eyes, which abut, but have not become united, as seen in other groups of Branchiopoda.[4]

The phenotypic plasticity shown by notostracan species make identification to the species level difficult.[8] Many putative species have been described based on morphological variation, such that by the 1950s, as many as 70 species were recognised.[8] Two important revisions – those of Linder in 1952[11] and Longhurst in 1955[12] – synonymised many taxa, and resulted in the recognition of only 11 species in the two genera. This taxonomy was accepted for decades,[8] "even attaining the status of dogma".[13] More recent studies, especially those employing molecular phylogenetics, have shown that the eleven currently recognised species conceal a greater number of reproductively isolated populations.[8]